The present invention relates to nuclear power and particularly to the control of recirculating steam generators in pressurized water nuclear steam supply systems. More specifically, the present invention is directed to automatic low power water level control for such steam generators.
A major contributing factor to nuclear plant unavailability is both high and low steam generator level trips during low power operation. This problem is particularly acute during plant start-up, when the operators have had relatively little experience in steam generator level control. The frequency of trips tends to fall off as the operator gains experience and confidence. However, because plants are often base-loaded, the operators have little opportunity to maintain their skills through varied actual operating experience at low power. Rather, training for low power operation is often dependent on plant simulators. Unfortunately, the ability of many of the operator training simulators today to accurately predict steam generator level dynamics, especially during low power operation, is rather limited. The lack of detailed process simulations in all but the most sophisticated simulators has prevented an accurate representation of this phenomena.
As a result, even experienced operators of pressurized nuclear power plants, have encountered difficulties in avoiding nuclear reactor trips during low power operation. This difficulty arises in part from the counter-intuitive behavior of the plant in response to control actions which may be taken by the operator. In particular, the steam generator water level often initially decreases when the operator increases the feed water flow to the steam generator.
In co-pending U.S. patent application No. 879,893, filed June 30, 1986, now U.S. Pat. No. 4,777,009 and assigned to the owner of the present patent application, an automatic steam generator feedwater control system for a full power operating range is disclosed and claimed. The disclosure of said copending application is hereby incorporated by reference. Such a system, although effective, is not well adapted for economical retro-fitting in existing nuclear power plants because of the delicate transitions among three power-operating regimes: a low power regime, typically in the range of 0-15% power, an intermediate regime in the range of 15-50% power, and a high power regime in the range of 50-100% power.
In the system of the '009 patent, the steam generator water level is controlled as a function of a flow demand signal wherein the feedwater pump speed demand is constant at low power and is modulated by the flow demand signal at intermediate and high power; the bypass valve position is modulated by the flow demand signal at low power, closed at intermediate power and open at high power; and the main valve position is closed at low power and modulated by the flow demand signal at intermediate and high power. The measured flow demand signal is first processed by an adaptive network in which the control coefficients are functions of the reactor power.